/Engine/Source/Runtime/Core/Public/Math/UnrealMathUtility.h
The implementation uses approximations for Exp/Sin/Cos. These are accurate for all sensible values of DampingRatio and InSmoothingTime so long as InDeltaTime < 0.5 * InSmoothingTime, but are generally well behaved even for larger timesteps etc.
template<class T> static void SpringDamperSmoothing ( T & InOutValue, T & InOutValueRate, const T & InTargetValue, const T & InTargetValueRate, const float InDeltaTime, const float InSmoothingTime, const float InDampingRatio )